Computer Simulation of Cyclic Block Copolymer Microphase Separation

Block copolymer systems: from single chain to self-assembled nanostructures

Recent advances in the field of macromolecular engineering applied to the fabrication of nanostructured materials using block copolymer chains as elementary building blocks are described in this feature article. By highlighting some of our work in the area and accounting for the contribution of other groups, we discuss the relationship between the physical-chemical properties of copolymer chains and the characteristics of nano-objects originating from their self-assembly in solution and in bulk, with emphasis on convenient strategies that allow for the control of composition, functionality, and topology at different levels of sophistication. In the case of micellar nanoparticles in solution, in particular, we present approaches leading to morphology selection via macromolecular architectural design, the functionalization of external solvent-philic shells with biomolecules (polysaccharides and proteins), and the maximization of micelle loading capacity by the suitable choice of solvent-phobic polymer segments. The fabrication of nanomaterials mediated by thin block copolymer films is also discussed. In this case, we emphasize the development of novel polymer chain manipulation strategies that ultimately allow for the preparation of precisely positioned nanodomains with a reduced number of defects via block-selective chemical reactivity. The challenges facing the soft matter community, the urgent demand to convert huge public and private investments into consumer products, and future possible directions in the field are also considered herein.

Brownian molecular dynamics simulation on self-assembly behavior of diblock copolymers: influence of chain conformation

Brownian molecular dynamic simulations are applied on the self-assembly behavior of AB-type diblock copolymers. The influence of chain conformation of core-forming A-block changing from rigid to flexible on the aggregation structure formed by AB copolymers is investigated. It is found that at a high rigid fraction f(R) of A-block, a disk structure can be formed at a high aggregation interaction epsilon(AA) of A-bead pairs because of the tendency of orientational packing of rigid portion within the aggregate core. Transitions of aggregation structure from disk to string, further to small aggregates, and to unimers are observed with decreasing epsilon(AA). The packing of A-blocks becomes more random at relatively lower values of f(R), resulting in the formation of spherical structure. The region of string becomes narrower while the regions of the small aggregates and sphere become wider as decreasing f(R). Meanwhile, the onsets of string, disk, and sphere formation move to higher epsilon(AA). The phase diagrams for the influences of rigid potion location within the A-block and the chain rigidity of the A-block are mapped. The comparison of simulation results with existing experimental observations is also presented. Our simulation results tend to bridge a gap of different micellization behaviors between rod-coil block copolymers and coil-coil block copolymers and extend to investigate chain conformation influence on phase diagram.

Multicompartment micelles and vesicles from pi-shaped ABC block copolymers: a dissipative particle dynamics study

Dissipative particle dynamics simulations were performed on the morphology and structure of multicompartment micelles and vesicles formed from pi-shaped ABC block copolymers in water. By varying the chain architecture and the composition of copolymers, a rich variety of morphologies were observed, such as oblate vesicles, trumpet vesicles, layered ribbon-like micelles and Y-shaped micelles. The simulations show that the hydrophilic block length and the distance between the two grafts play important roles in the control of the morphology. Since pi-shaped ABC block copolymers can reduce to linear ABC and star ABC block copolymers, they are good model copolymers for studying the self-assembly of complex block copolymers into micelles or vesicles. Thus, the knowledge obtained in this work as well as the new morphologies identified provides useful information for future rational design and synthesis of novel multicompartment micelles and vesicles.

Microphase separation of diblock copolymer poly(styrene-b-isoprene): A dissipative particle dynamics simulation study

Dissipative particle dynamics (DPD) simulations have been employed to study the microphase separation of the poly(styrene-b-isoprene) (PS-b-PI) diblock copolymer. The DPD model is constructed to match the physical description and structural properties of the PS-b-PI diblock copolymer. A coarse-grained force field has been developed for the diblock copolymer system in DPD simulations. The new force field contains bonded and nonbonded interaction terms, which are derived from atomistic molecular dynamics simulations and determined by fitting experimental data of the compressibility of water at room temperature and interfacial tension values, respectively. The morphologies of the PS-b-PI diblock copolymer system obtained from DPD simulations are in agreement with experimental observations as well as previous simulated results.

Self-Organization Process of Ordered Structures in Linear and Star Poly(styrene)−Poly(isoprene) Block Copolymers: Gaussian Models and Mesoscopic Parameters of Polymeric Systems

Mesoscopic simulations of linear and 3-arm star poly(styrene)-poly(isoprene) block copolymers was performed using a representation of the polymeric molecular structures by means of Gaussian models. The systems were represented by a group of spherical beads connected by harmonic springs; each bead corresponds to a segment of the block chain. The quantitative estimation for the bead-bead interaction of each system was calculated using a Flory-Huggins modified thermodynamical model. The Gaussian models together with dissipative particle dynamics (DPD) were employed to explore the self-organization process of ordered structures in these polymeric systems. These mesoscopic simulations for linear and 3-arm star block copolymers predict microphase separation, order-disorder transition, and self-assembly of the ordered structures with specific morphologies such as body-centered-cubic (BCC), hexagonal packed cylinders (HPC), hexagonal perforated layers (HPL), alternating lamellar (LAM), and ordered bicontinuous double diamond (OBDD) phases. The agreement between our simulations and experimental results validate the Gaussian chain models and mesoscopic parameters used for these polymers and allow describing complex macromolecular structures of soft condensed matter with large molecular weight at the statistical segment level.

The dependence of nanostructures on the molecule rigidity of A2(B4)2-type miktoarm block copolymer

Using the dissipative particle dynamics simulation technique, we have studied the influence of the molecule rigidity on the nanostructures of the A2(B4)2-type miktoarm block copolymers. A typical spherical micellar ordered structure is obtained for a coil-coil miktoarm block copolymer in melt. By introducing a bond angle potential in our model to enhance the molecule rigidity systematically, we find, respectively, a hexagonal cylindrical structure and a parallel ellipsoid in lamellae structure which is discovered for the first time.

Aggregation of Poly(ethylene oxide)−Poly(propylene oxide) Block Copolymers in Aqueous Solution: DPD Simulation Study

The dissipative particle dynamics (DPD) simulation method was applied to simulate the aggregation behavior of three block copolymers, (EO)16(PO)18, (EO)8(PO)18(EO)8, and (PO)9(EO)16(PO)9, in aqueous solutions. The results showed that the size of the micelle increased with increasing concentration. The diblock copolymer (EO)16(PO)18 would form an intercluster micelle at a certain concentration range, besides the traditional aggregates (spherical micelle, cylindrical micelle, and lamellar phase); while the triblock copolymer (EO)8(PO)18(EO)8 would form a spherical micelle, cylindrical micelle, and lamellar phase with increasing concentration, and (PO)9(EO)16(PO)9 would form intercluster aggregates, as well as a spherical micelle and gel. New mechanisms were given to explain the two kinds of intercluster micelle formed by the different copolymers. It is deduced from the end-to-end distance that the morphologies of the diblock copolymer and triblock copolymer with hydrophilic ends were more extendible than the triblock copolymer with hydrophobic ends.